The 12-volt systems used in today's automobiles are required to supply ever increasing currents as the load on the system continues to increase. This increase is due to a combination of increasing numbers of electronic devices, such as communication, entertainment, and telematics systems, as well as the proliferation of electric powered auxiliary systems to replace traditional hydraulic or mechanical powered systems. To reduce the amount of current required to supply these higher loads, it has been proposed that automobiles should adopt 42-volt electrical systems. The automotive industry, however, has been reluctant to transition to 42-volt electrical systems because of increased costs. Consequently, there is a strong demand to improve the performance of 12-volt systems, thereby allowing higher electrical loads to operate effectively with conventional vehicle electrical systems.
The available output current from typical automotive alternators is determined in significant part by the speed at which the alternator is operating. Because alternators are driven off the engine, the alternator operating speed is a direct function of engine speed. For example, an alternator that produces a current of 135 amperes at an engine speed of 3,000 rpm may only produce 60 amperes at an engine idle speed of 600 rpm.
Most electrical loads in an automobile are insensitive to engine speed, such as rear window defoggers, heated seats, lights, etc. Conversely, the automotive electrical loads that are sensitive to engine speed do not consume significant current, for example, the ignition system. As a result, an automotive electrical system may be in significant current deficit when the engine is at idle. This current deficit may result in temporary voltage dips on the 12-volt bus. When this voltage dip occurs, a variety of objectionable performance is experienced from various electrical systems, for example dimming of the vehicle lights.
A variety of solutions to this problem in vehicle electrical system have been proposed. For example, U.S. Pat. No. 5,973,482 describes a voltage regulator that is fed by a standard boost converter circuit and produces the desired increase in field voltage. The number of components in the required circuit, however, increases the cost of the circuit rendering it less desirable than the circuits disclosed herein. Additionally, the feedback loop of the '482 patent is more complicated than the circuits described herein because the boost output voltage is fed back to the boost control, while regulation of the output is still performed by the regulator.
U.S. Pat. No. 4,410,848 describes another solution to maintaining alternator excitation. The alternator disclosed in the '848 patent is a three phase AC output without rectifier diodes. The field winding is driven by the rectified output voltage. Then a large load is applied, the output voltage collapses, thereby collapsing the field voltage as well. The '848 patent discloses an arrangement of current transformers to maintain field current when the output voltage collapses. This system requires additional components (current transformers) rather than the boost circuits described herein, which increases cost of the solution and is therefore less desirable.
Finally, U.S. Pat. No. 5,946,202 describes techniques for boosting alternator output using switches (e.g., field effect transistors (FETs)) to short the alternator outputs to ground at high frequencies, thereby using the line inductance as an energy storage element. All of the alternator output power is switched through the three FETs, which replace the rectifier diodes. Again, this solution requires substantial additional cost and complexity over the circuitry described herein.
Thus, disclosed herein are circuits and methods for minimizing the above-mentioned drawbacks and while still solving or at least minimizing the problems of maintaining alternator voltage.